In 3GPP Long-Term Evolution (LTE) networks, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipment (UEs). Orthogonal Frequency Division Multiple Access (OFDMA) has been selected for LTE downlink (DL) radio access scheme due to its robustness to multipath fading, higher spectral efficiency, and bandwidth scalability. Multiple access in the downlink is achieved by assigning different sub-bands (i.e., groups of subcarriers, denoted as resource blocks (RBs)) of the system bandwidth to individual users based on their existing channel condition.
The initial access by a UE to a cell served by an eNB involves a number of steps. In step 1, the UE first acquires LTE rel-8 PSS/SSS (primary synchronization signal and/or secondary synchronization signal) to synchronize to the cell within an acceptable range of residual Carrier Frequency Offset (CFO) and residual timing offset. The UE also acquires cell ID of the cell. In step 2, to ensure there is no erroneous synchronization, the UE detects the Master Information Block (MIB) on PBCCH (Physical Broadcast Control Channel). If CRC is valid, the UE acquires basic system information such as System Frame Number (SFN) broadcasted in the MIB for frame timing and fine tune synchronization parameters using LTE rel-8 Cell Specific Reference Signals (CRS).
The initial access can take up to 600 ms at very low signal to noise ratio (SNR). To achieve an overall coverage enhancement target of 20 dB, even longer access times are needed, e.g., two seconds for FDD system and possibly longer for TDD system. For MTC (machine type communication) devices, after initial access to a cell, a machine may go to sleep for a very long time (i.e., several minutes) to conserve energy. The machine may lose synchronization with the cell completely after waking up. Because rel-8 CRS are typically used with post-FFT detector for fine synchronization or synchronization tracking with reasonable residual CFO (to limit inter-carrier interference) and residual timing offset (FFT window timing), using rel-8 CRS to re-synchronize with the cell may not be possible. As a result, the machine may need to use PSS/SSS to re-synchronize with the cell, which adds to latency and power consumption.
A machine-specific pilot “mSYNC” with higher density in time and frequency domain transmitted with a relatively longer periodicity (i.e., >>5 ms) for fast timing and frequency acquisition and tracking could help improve latency and power consumption for machine re-synchronizing or tracking synchronization. A solution is desired to allow machine to wake up just before the mSYNC is scheduled by eNB to avoid excessive power consumption. The higher time-frequency density of mSYNC is designed to allow machine to re-synchronize or track synchronization with the network without the need for long averaging time, which improves latency.